The present invention is directed at remedying the problem in gas turbine engines wherein the tips of the turbine blades of the engine penetrate the linings of the shroud segments which surround them and thereby destroy the desired clearance therebetween with resulting loss in efficiency in certain flight conditions. Various attempts have been made at remedying the problem of controlling radial growth of the casing about the turbine blades during take-off and other transient operating conditions of the engine where the difference between blade tip and casing growth is greater. During transient conditions it is desirable to keep the casing hot whereas in steady state conditions, it is desired to cool the casing.
Former solutions to attempt to control the gap between the blade tips and the shroud segments have involved the use of large mechanical valves and piping which had to be accommodated in the spaces provided in the engine compartment. These valves and associated piping were very large in size and accordingly occupied critical spaces and provided added weight and cost to the engine.
In some of the attempts to remedy the above-noted problem some have resorted to the use of proportioners which utilize metering devices which permit the supply of hot or cold or mixed air to the shroud plenum. Such a device is for example described and illustrated in U.S. Pat. No. 5,064,343 issued on Nov. 12, 1991. The proportioner as illustrated therein controls the amount of hot or cold air going to the plenum above the rotor shroud in order to control tip clearance with engine conditions. This proportioner relies on hot gas temperatures for thermal radial expansion relative to the other static parts which follow the cold air supply temperature. It is this thermal mismatch, combined with appropriate discrete holes which permits metering of hot or cold or mixed air supply to the shroud plenum. The proportioner is a U-shaped ring which moves in and out radially in a slot which implies fretting and possible loss of sealing surface with the static parts. With time, cold air leakage seems unavoidable and this compromises the function of the system.
In U.S. Pat. No. 3,966,354 there is also proposed a thermal actuated valve for clearance control using bleed air from the compressor to supply hot or cooler air to heat or cool the shroud. Their passive thermal valve bypasses cooler air and admits hot air against the shroud from the bleed conduits. The reaction time of expansion and contraction of the shroud is slow in comparison with the reaction time of the rotor blades. The structure proposed also occupies valuable space about the shroud.